A
Good Definition of the Word "Planet": Mission Impossible?

5)
Runts and Runaways

Indeed, one alteration
of orbits can be the outright ejection of a planet. When several big planets
are orbiting a star under each other's influence, the system is inherently unstable.
The almost inevitable result is that the smallest body will be ejected from
the system. We would see it as an isolated object smaller (less massive) than
a brown dwarf. Such objects have indeed been found in some young star clusters.
The problem is, we are not sure whether such small objects can also just form
by themselves, without ever being in orbit around a star. Originally theorists
claimed this was not possible, but as more details were added, they retreated.
We know of no good reason why the smallest objects with fusion should have the
same mass as the smallest objects that can form by themselves. At the moment
it seems more likely that the discoveries are not true ejected planets (but
it is very hard to be sure).

A controversy therefore
arose when they were dubbed "free-floating planets" by some of the
discoverers. The question is: if the object was never in orbit around a star,
can we call it a planet? Some astronomers say that if it never had fusion then
it clearly isn't a star (failed or otherwise). Since the free-floating objects
have the same mass as some of the accepted extrasolar planets, they should be
called planets too. Others say that planets can only form and be found around
stars (leaving aside the problem of ejection), so if the new objects formed
in isolation they should be called "sub-brown dwarfs" or "grey
dwarfs", but certainly not "planets".

The latest computer
simulations of star formation only further confuse the issue. They show that
in the formation of a cluster of stars, fetal objects are often interacting
with each other, forming loose alliances, then being ejected from the group
while formation is still in progress. Brown dwarfs sometimes form by themselves.
Sometimes they are part of a multiple star system in the process of formation,
when they are suddenly tossed out (robbing them of their "rightful"
supply of gas). The same could be true of objects in the planetary mass range,
in which case it is hard to say whether they formed in orbit around a star or
not. (For more information on these computer models, try this link: http://www.astro.ex.ac.uk/people/mbate/Research/pr.html)

By now you should
be at least as confused as professional astronomers are. What seemed like an
easy question ("what is a planet?") has become a morass. Still, the
word is in very common use, and it would be nice to know what we are talking
about. A final definition should be acceptable to both scientists and the lay
public. Your mission, should you choose to accept it, is to concoct such
a definition. There are three arenas from which the definition could spring.
They are not necessarily compatible with each other, and one or more may not
be necessary. These are 1) the characteristics of the objects themselves;
2) the circumstances in which they are found; and 3) their cosmogony
(how they form).

I here suggest
a few properties that the definition perhaps should satisfy, but you are free
to add to and subtract from this list at will. A good way to start is probably
to settle on the list for yourself. My suggestions (in no particular order)
are that a definition should 1) be physical: give some fundamental properties
of the object; 2) be observable: depend on measurements that are feasible
to accomplish; 3) be succinct and clear, with little ambiguity; 4) be
general beyond current observations: allow room for new discoveries;
5) have well-defined limits: except right at these limits it should be
easy to place an object inside or outside the category; and 6) be easily
understood by the public but satisfactory to scientists.

The International
Astronomical Union (the only body empowered to make an "official"
definition), has found the task problematic so far. Nature, of course, cares
nothing about classification, and the truth is that there is a continuum of
characteristics, circumstances, and probably cosmogonies to the objects out
there. This article contains the beginnings of many of the relevant issues.
Have the students extract them (and perhaps augment them with their research),
and organize them so to aid in carrying out the mission. Can they bring clarity
to the classification and cut through the confusion? Can they make a definition
for "planet" that other students will like and understand, and that
teachers and astronomers will also find compelling? It should at least be fun
and informative to try.

I don't think this
mission is impossible, and will suggest a definition in a future article in
Mercury. Or you can get a preview at my website (http://astro.berkeley.edu/~basri/whatsaplanet.htm)
after you have finished your own thought process. This document will not self-destruct
anytime soon, and you are free to spread it around. Good luck!